In 1984, Charles Bennet et al. devised a quantum cryptographic protocol and the like. The quantum cryptography has triggered vigorous studies intended to achieve quantum cryptographic communications. In particular, the generation of a pair of quantum-correlated photons (i.e., a pair of quantum-entangled photons) is essential to a cryptographic protocol based on the nonlocal correlation between paired photons and a quantum transponder based on the principle of quantum teleportation, and therefore plays a very important role as an elemental technology for optical quantum cryptographic communications.
Thanks to the development of a method based on parametric downward conversion, it has become possible to generate a pair of quantum-correlated photons with some degree of efficiency. This method includes irradiating a nonlinear optical crystal with a high-power laser beam and obtaining a pair of photons through a secondary nonlinear optical process. This method has been used to date to conduct application experiments. However, this method presents such drawbacks as follows: (i) it is not efficient in generation; causes a difficulty in generating a pair of short-wavelength photons; and (iii) it causes a difficulty in generating multiphotons. Therefore, there has been a need for a new method for generation using a semiconductor or the like.
In 2004, under such circumstances, Keiichi Edamatsu et al. achieved the generation of a pair of short-wavelength correlated photons through a resonant hyper-parametric scattering process in a semiconductor CuCl bulk crystal (see Non-patent Literature 1 and Patent Literature 1). Further, Hiroshi Ajiki et al. proposed a structure in which a resonator has a semiconductor nanocrystal embedded therein, and were able to increase the efficiency of generation of a pair of photons several digits by using the structure (see Non-patent Literature 2 and Patent Literature 2).
As compared with the conventional method based on parametric downward conversion, the generation of a pair of correlated photons through resonant hyper-parametric scattering in a semiconductor bulk crystal allows high-efficiency generation by using the resonance levels of electrons, and its greatest feature and merit lie in the fact that a pair of short-wavelength correlated photons is obtained. Ajiki et al. proposed generating a pair of correlated photons more highly efficiently by using a semiconductor nanocrystal, which is more desirable in terms of being made into a device, while keeping such an advantage.
According to this proposal, a pair of correlated photons is taken out by a method for active use of a resonator polariton state (mixed state of an exciton and a resonator photon) that is formed in a resonator. First, two types of excitation light are allowed to be concentrically incident upon a sample perpendicularly. The wavelength of one of the excitation beams is adjusted to be a wavelength corresponding to that of the lower branch of a resonator polariton, and the wavelength of the other beam is adjusted to be another wavelength that is determined by a parameter of the sample. Thus, the dual-wavelength light is used as energy for generating a dual-particle excited state. Paired photons thus generated are emitted at their respective predetermined angles of emission in such directions as to be symmetrical about a direction normal to a surface of emission. The paired photons thus emitted are detected by detectors provided in positions corresponding to the respective angles of emission.